The present invention generally relates to a platen/polishing pad assembly for chemical mechanical polishing and a method for using and more particularly, relates to a ventilated platen/polishing pad assembly for chemical mechanical polishing and a method for using such ventilated platen/polishing pad assembly for achieving more uniform polishing.
Method and apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head rotates and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing a metal oxide may be formed and removed repeatedly.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan(copyright) head which differs from conventional CMP heads in two major respects is shown. First, the Titan(copyright) head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
The mechanism for chemical mechanical polishing of metal is different and more complex than the polishing of silicon oxide. It is generally believed that during the CMP of metal, metal form an oxide layer on the surface which is subsequently removed by the polishing pad by a mechanism similar to that for oxide polishing. For instance, a mechanism that involves hydroxylation, bond formation with slurry and then, bond breaking from wafer. After the metal oxide layer is removed from the metal surface, metal is etched by the chemicals in the slurry solution, while simultaneously the exposed metal forms a new passivation layer through oxidation by the slurry solution. In practice, it is believed that three processes, i.e. the removal of metal oxide, the metal etching and the metal passivation occur simultaneously. A polishing slurry solution for use in metal CMP therefore contains three major components of fine slurry particles, a corrosion or etchant agent and an oxidant. The eventual planarization of the metal surface is achieved by the rigidity and planarity of the polishing pad similar to a process of oxide polishing.
When the metal being polished in the CMP process is copper, the polishing process becomes more complicated due to the characteristics of copper. Since copper is frequently used in multi-level interconnect structures in semiconductor devices, i.e. in damascene or dual damascene structures, a CMP step for forming copper interconnects in the damascene structures with satisfactory polishing uniformity becomes an important link in the entire fabrication process. The copper CMP process produces a fresh copper surface which is susceptible to corrosion caused by a photolytic reaction.
It is therefore an object of the present invention to provide a ventilated platen/polishing pad assembly for use in chemical mechanical polishing of copper conductors on a semiconductor wafer that does not have the drawbacks or shortcomings of the conventional polishing platen.
It is another object of the present invention to provide a ventilated platen/polishing pad assembly for use in chemical mechanical polishing copper conductors on a semiconductor wafer that does not have copper corrosion and polishing non-uniformity problems.
It is a further object of the present invention to provide a ventilated platen/polishing pad assembly for use in chemical mechanical polishing copper conductors on a semiconductor wafer for flowing an oxidizing gas through the ventilated platen/polishing pad assembly such that the oxidizing gas mixes with a slurry solution for polishing the copper surface.
It is another further object of the present invention to provide a ventilated platen/polishing pad assembly for use in chemical mechanical polishing copper conductors on a semiconductor wafer by flowing an oxidizing gas into a slurry delivery conduit forming a slurry/oxidizing gas mixture for polishing the copper surface.
It is still another object of the present invention to provide a ventilated platen/polishing pad assembly for use in chemical mechanical polishing copper conductors on a semiconductor wafer wherein the platen is provided with a multiplicity of recessed surface grooves and apertures for flowing an oxidizing gas therethrough.
It is yet another object of the present invention to provide a chemical mechanical polishing apparatus which incorporates the use of a ventilated platen/polishing pad assembly for mixing an oxidizing gas with a polishing slurry to prevent fresh copper surfaces from corrosion by the acidic components in the slurry solution.
It is still another further object of the present invention to provide a method for chemical mechanical polishing a semiconductor wafer by flowing an oxidizing gas through a ventilated platens/polishing pad assembly forming a slurry/oxidizing gas mixture for polishing a copper surface without corrosion problems.
It is yet another further object of the present invention to provide a method for chemical mechanical polishing copper conductors on a semiconductor wafer by flowing an oxidizing gas into a slurry delivery conduit forming a slurry/oxidizing gas mixture for use in the polishing process.
In accordance with the present invention, a ventilated platen/polishing pad assembly for chemical mechanical polishing copper conductors on a semiconductor wafer and a method for using such assembly are disclosed.
In a preferred embodiment, a ventilated platen/polishing pad assembly for chemical mechanical polishing copper conductors on a semiconductor wafer is provided which includes a platen of circular shape that is made of a rigid material having a first thickness, a top surface and a bottom surface; a first plurality of apertures through the first thickness which provides fluid communication between a gas inlet provided on the bottom surface and a first plurality of openings provided in a top surface of the platen; and a polishing pad that has a second multiplicity of apertures therethrough for providing fluid communication between a second multiplicity of openings in a top surface of the polishing pad and the first plurality of openings in the top surface of the platen when the polishing pad is assembled to the top surface of the platen.
In the ventilated platen/polishing pad assembly for chemical mechanical polishing copper conductors, the top surface of the platen further includes a multiplicity of recessed grooves to facilitate the flow of a gas from the gas inlet to the second multiplicity of openings in the top surface of the polishing pad. Each of the multiplicity of recessed grooves is in fluid communication with at least one of the second multiplicity of apertures in the polishing pad. The polishing pad further includes at least two layers of pads formed of at least two different materials situated in the thickness direction of the pad. The at least two different materials have different hardness. The at least two layer of pads include a bottom layer for bonding to the platen which has a first hardness and a top layer exposed which has a second hardness, the second hardness is smaller than the first hardness, The polishing pad may be assembled to the platen by adhesive means, or by a pressure-sensitive adhesive.
The present invention is further directed to a chemical mechanical polishing apparatus which includes a ventilated platen/polishing pad assembly mounted on a rotatable shaft, the assembly includes a platen of circular shape that is made of a rigid material with a first thickness, a top surface and a bottom surface; a first multiplicity of apertures through the first thickness providing fluid communication between a gas inlet provided on the bottom surface and the first multiplicity of openings provided in the top surface of the platen; and a polishing pad that has a second multiplicity of apertures therethrough for providing fluid communication between a second multiplicity of openings in a top surface of the polishing pad and the first multiplicity of openings in the top surface of the platen when the polishing pad is assembled to the top surface of the platen; a wafer holder for holding a wafer and contacting an active surface of the wafer with the top surface of the polishing pad; motor means for rotating the ventilated platen/polishing pad assembly and the wafer holder in opposite directions; a dispenser means for dispensing a slurry solution onto the top surface of the polishing pad; and an enclosure for enclosing the ventilated platen/polishing pad assembly, the wafer holder, the motor means and the dispensing means.
In the chemical mechanical polishing apparatus, the top surface of the platen further includes a multiplicity of recessed grooves to facilitate the flow of a gas from the gas inlet to the second multiplicity of openings in the top surface of the polishing pad. Each of the multiplicity of recessed grooves is in fluid communication with at least one of the second multiplicity of apertures in the polishing pad. The polishing pad may include at least two layers of pads formed of at least two different materials situated in the thickness direction of the pad. The chemical mechanical polishing apparatus may further include a conditioning arm incorporating a conditioning disc mounted thereon for conditioning the top surface of the polishing pad.
The present invention is further directed to a method for chemical mechanical polishing a semi-conductor wafer which includes the steps of providing a platen/polishing pad assembly mounted on a rotatable shaft; mounting a wafer in a wafer holder with a surface to be polished exposed; rotating the wafer surface to be polished in contact with and against a top surface of the polishing pad; and dispensing a polishing slurry/oxidizing gas mixture in between the wafer surface and the top surface of the polishing pad.
The method for chemical mechanical polishing copper conductors on a semiconductor wafer may further include the step of forming the polishing slurry/oxidizing gas mixture by injecting the oxidizing gas into the polishing slurry. The method may further include the step of injecting the oxidizing gas into a polishing slurry delivery conduit before the solution is dispensed from the delivery conduit.
The method for chemical mechanical polishing copper conductors on a semiconductor wafer may further include the steps of providing a first multiplicity of apertures through the platen of the platen/polishing pad assembly, providing a second multiplicity of apertures through the polishing pad of the platen/polishing pad assembly, and flowing an oxidizing gas through the first and the second multiplicity of apertures and mixing the oxidizing gas with a polishing slurry solution.
The method may further include a step of forming the polishing slurry/oxidizing gas mixture by at least one oxidizing gas selected from the group consisting of NO, N2O, O3 and O2 The method may further include the step of polishing a copper layer on the wafer surface.